Rotary compressors generally are capable of reverse operation wherein they act as expanders. Reverse operation can occur at shutdown when the closed system seeks to equalize pressure via the compressor thereby causing the compressor to run as an expander with negligible load. This problem has been addressed by providing a discharge check valve, as exemplified by commonly assigned U.S. Pat. No. 4,904,165, wherein the check valve is located as close as possible to the scroll discharge to minimize the amount of high pressure gas available to power reverse operation. As long as any high pressure gas is available to power reverse operation, some movement of the orbiting scroll will take place with attendant noise even if there is no attendant danger to the scroll compressor. Even if not harmful, the noise can be annoying and its reduction and/or elimination is desirable.
Scroll compressors in addition to tending to run in a reverse direction at shutdown also self unload at shutdown. The scrolls must be held in sealing contact in opposition to the forces exerted by the gas being compressed. Typically, the axial forces tending to hold the scrolls in contact, the axial compliancy, is supplied by fluid pressure acting against a scroll member from one or more pockets supplied with discharge and/or intermediate pressure. Leakage from the pockets(s) normally coacting with gravity axially separates the scrolls to provide leakage at the wrap tips thereby unloading the compressor, if not already unloaded, independent of radial movement of the scrolls due to gas forces acting on the scroll or gravity causing leakage at the wrap flanks and thereby unloading the compressor. Thus, scroll compressors are inherently unloaded a short while after stopping and remain unloaded until restarted and thereby have an easy start since they do not have to start against a pressure head. In contrast, other compressors generally are not self unloading except where reverse operation takes place with its attendant problems. As a result, it is common to unload reciprocating compressors, for example, at shutdown or start up in order to have an easy start. This approach is exemplified by U.S. Pat. Nos. 2,039,089; 2,579,439; and 2,715,992. Unloading and the use of variable speed for capacity control are well known. Thus, scroll compressors are unloaded only as part of a continuing operation responsive to demand or inherently as a consequence of stopping the compressor. Scroll compressors are not unloaded prior to shutoff as a part of the shutting off procedure or at shutoff by providing preferential bypass.